Ligand-to-metal a donation

Further improvements in activity of the imidazol-2-ylidene Ru complexes might be attained by the incorporation of a better a-donor substituents with larger steric requirements. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (14 e") of the ruthenium metallacyclobutane intermediate [7]. Both ligand-to-metal a-donation and bulkiness of the NHC force the active orientation of the carbene moiety and thus contribute to the rapid transformation into metallacyclobutane species [7b]. Both can be realized by incorporation of alkyl groups in 3,4-position of imidazol-2-ylidene moiety, lyie Me. Me... 

Figure 3.3. The synergistic stabilizing effect of metal carbonyl complexes. Shown is (a) ligand-to-metal O donation from the carbon lone pair to the metal d 2 orbital and (b) metal-to-ligand back-donation from the d 2 y2 orbital to the empty TZ orbital on CO. This weakens the C-O bond, while concomitantly strengthening the M-C interaction.

The overlap between the metal and ligand orbitals in the 5i>2g orbital (LUMO for Z = 0) provides an efficient pathway for ligand-to-metal electron donation and confers electrophilic character on the ligands in neutral complexes. This finding affords a reasonable explanation for the electrophilic mechanism proposed by Wang and Steifel for the nonclassical reaction of neutral Ni-bis(dilhiolene) complexes wilh olefins, which can be separated and purified by a novel melhod. The LUMO of Ihe dilhiolene complex... 

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